The present invention relates to a gas sensor interface device for interfacing a gas sensor to external equipment and a gas sensor system equipped with the gas sensor interface device.
Under present circumstances where automotive emission regulations are becoming strict year after year, there is a demand to provide an internal combustion engine with a high-precision exhaust gas sensor and sensor control system for accurate engine air-fuel ratio control. In the case of a gasoline vehicle, the internal combustion engine commonly employs a three-way catalyst and, when operated in stoichiometric conditions, achieves maximum catalyst efficiency to clean deleterious exhaust gas highly efficiently. It is thus particularly desirable, as a low-emission control technique, to control an air-fuel ratio of the engine with no or less deviation from a stoichiometric air-fuel ratio value.
One example of the exhaust gas sensor is oxygen sensors. The oxygen sensors can be classified into two types: one type is a so-called “γ sensor” capable of producing a binary output according to the concentration of oxygen in exhaust gas, i.e., whether the air-fuel ratio is on the rich side or lean side of stoichiometry; and the other is a full-range sensor capable of producing an output with a certain degree of linearity over a wide oxygen concentration range. The full-range oxygen sensor enables more accurate engine air-fuel ratio control. The full-range oxygen sensor also enables not only stoichiometric combustion control (feedback control of the air-fuel ratio to around a stoichiometric value) but also lean combustion control (feedback control of the air-fuel ratio within a lean range) accurately.
The output of the full-range oxygen sensor is generally in the form of a current signal responsive to the concentration of oxygen in exhaust gas. As the current signal of the oxygen sensor is transmitted to any external equipment via a gas sensor interface device, a current detector is arranged in the first circuit stage of the gas sensor interface device to detect the current signal from the full-range oxygen sensor. Each of Japanese Laid-Open Patent Publication No. 1-152356 and No. 2004-205488 proposes such a current detector circuit for use in the gas sensor interface device, including a detection resistor and a differential amplifier to generate an output voltage by amplifying a potential difference across the detection resistor upon receipt of the current output from the oxygen sensor.